In the production of silicon, silicon carbide or ferrosilicon, quartz sand, a carbon carrier and a bituminous binder can be mixed together and the mixture shaped into blanks from which green briquettes are formed.
As a rule, quartz sand in a range of grain sizes of from 0.05 to 0.2 mm is used for the preparation of briquettes of this kind, a variety of particle size ranges employed between these limits. Of course, for the preparation of ferrosilicon finely divided iron or finely divided iron oxide can also be added to the green briquettes.
Green briquettes for the purposes specified must comply with special chemical and physical requirements. For example, and as can be gathered from U.K. Patent No. 2 084 122, an important chemical consideration in the preparation of silicon is that as the green briquettes pass through the low-shaft electric furnace, silicon carbide may first form in the top part in accordance with the equation: EQU SiO.sub.2 +3C=SiC+2CO
The total carbon content of the individual green briquettes must be adjusted correspondingly It is usually adjusted to a hyperstoichiometric level (cf. U.K. Patent No. 2 150 128). From the physical angle it must be ensured that, as the green briquettes pass through the furnace, they do not disintegrate, especially during the reduction to SiC, in order to react in the bottom part of the furnace with molten quartz of the charge in accordance with the equation: EQU SiO.sub.2 +SiC=3Si+2CO 2CO
Of course, other reactions also occur in the furnace. One of them is the formation in accordance with the formula: EQU SiO.sub.2 +C=SiO+CO
at the high furnace temperatures of silicon monoxide, some of which is lost, with detriment to the yield and the heat balance. In practice, the only possible way of preparing green briquettes for the production of silicon in a manner satisfying chemical and physical requirements has been to use hot briquetting (c.f. U.K. Patent No. 2 088 840).
This hot briquetting process produces, at temperatures around 500.degree. C., briquettes containing molten or fused caking coal as the bituminous binder. The melting point of this coal binder, which may be between 350.degree. and 450.degree. C., must be exceeded during briquetting if the molten coal is to be distributed satisfactorily in the material to be bound and if the blank is to be consolidated by the formation of semicoke.
A very important factor in briquette quality has been a special heat treatment in which the blanks are stored substantially at their briquetting temperature in insulated bunkers for several hours--i.e., a heat treatment or tempering and using the intrinsic heat of the blanks thereby doubling the strength of the briquettes.
However, it has been found that preparing green briquettes for the production of silicon or silicon carbide or ferrosilicon by hot briquetting is relatively costly.
Another disadvantage when there are stringent purity requirements for the products to be produced from the briquettes is that impurities may be taken up into the briquettes during hot briquetting either from the caking coal or from metal abrasion of the apparatus in which the hot briquetting is carried out. Similar considerations apply to the production of silicon carbide or ferrosilicon.
The term "ferrosilicon" comprises for the purposes of the invention a metal alloy containing silicon, more particularly, with iron as metal.